Differential amplifiers are commonly used for a variety of purposes, including for delay stages in analog delay lines. In such analog delay lines, a plurality of differential amplifiers are coupled in series from a first differential amplifier to a last differential amplifier. A differential signal applied to differential inputs of the first differential amplifier may be successively delayed as it propagates through each of the differential amplifiers. As a result, a differential signal that is produced at differential outputs of the last differential amplifier may have a phase or timing that is delayed relative to the differential signal that is applied to the input of the first differential amplifier.
In some cases, it may be desirable for the differential signal coupled through the differential amplifiers of an analog delay line to have an amplitude that is as small as possible. A signal with a small amplitude may cause transistors used in the differential amplifier delay stages to consume less power than would be consumed by propagating a differential signal through the differential amplifiers having a larger amplitude. Further, a differential signal having a smaller amplitude may cause the magnitude of a delay provided by an analog delay line to be less sensitive to changes in a supply voltage used to power the analog delay line. For example, in its extreme example, a differential signal having a peak-to-peak amplitude equal to the difference between two supply voltages may cause the delay to vary substantially with the magnitude of the supply voltages since the differential signal propagating through the delay line may transition between the two supply voltages.
In practice, it may be difficult to set the amplitude of the differential signal to a minimum value that will propagate through the analog delay stages. Even if a differential signal having a small amplitude is applied to the first differential amplifier, if the differential amplifiers used as the delay stages have too much gain, the amplitude of the differential signal propagated through the delay line may progressively increase until it reaches a level that may result in excessive power consumption and sensitivity to power supply voltage changes. On the other hand, if the gain of the differential amplifier is too small, the differential signal propagated through the delay line may progressively decrease until it disappears. It may therefore be desirable to control the gain of differential amplifiers used in analog delay lines and other circuits to a fairly precise value.
The gain of one commonly used differential amplifier using a pair of differential input transistors is proportional to the transconductance of the transistors, which is often abbreviated as “gm.” More specifically, in many such amplifiers, the gain is the product of gm and the impedance of a load, which may be a transistor or resistance. If the load is a resistance that does not change with such factors as semiconductor process variations, supply voltage variations, or temperature variations, the gain of a differential amplifier may be proportional to only the gm of the differential input transistors. If the gm of the differential input transistors can be precisely controlled so that the differential amplifier has a substantially unity gain, the signal propagated through an analog delay line using such differential amplifiers as its delay stages may have a minimum amplitude, thereby allowing the delay line to consume relatively little power and to be substantially insensitive to power supply and other variations.
Unfortunately, it may be difficult to control the gain of differential amplifiers in the presence of process, supply voltage and temperature variations. As a result, it may be necessary to apply a differential input signal to the first stage of an analog delay line having an amplitude that is large enough to ensure propagation through the delay line assuming worst case gains of the differential amplifier stages. The result may be a less than ideal power consumption and supply voltage susceptibility of the delay line.